1. Field of Invention
The invention relates to electric power generation, and, more particularly, to bulk energy storage and frequency regulation of electrical power.
2. Description of Prior Art
Energy storage has been an important component of efficient power grid operation. As more renewable energy, particularly wind and solar photovoltaic, is integrated into electric grids, the intermittent and/or periodic nature of these energy resources may be poorly matched to the load pattern on the grid, which can limit the deployment of renewable energy systems. Efficient bulk energy storage enables both a shifting of the delivery time and the amount of energy in order to better match a grid's load pattern.
Various approaches are used for the storage of electric power, including battery, flywheel, and pumped storage devices, along with systems using thermal energy, gravity, buoyancy and compressed air. The approaches present a combination of low efficiency, high capital costs, high energy costs, and/or negative impacts to the environment. For example, some systems utilize masses stored at a particular height, which later descends, to harness the potential energy. Such systems are not appropriate for frequency regulation because of the delay caused by reaction time.
Flywheel systems deployed today operate at very high rotational speeds, which limits the power capacity of these systems due to the fact that known materials have limited strength when subjected to very high centrifugal forces and hoop stresses. These technical constraints limit current flywheel systems to power capacities of no more than a few megawatts per unit. Utilities need systems that can provide 20 or more megawatts of power over a period of time ranging from a few seconds to over 15 minutes. To do so, flywheel installations are presently comprised of multiple small units, increasing the complexity and cost of these systems.
What is needed is bulk (multi-megawatt) energy storage, which is cost effective, easily deployed, and responsive to the rapidly changing needs of either having power drawn from the grid to increase energy in storage, or having energy withdrawn from storage to the grid. The storage system also needs to have minimal mechanical and electrical system losses, as well as minimal windage losses.